It is desirable to minimize the bandwidth of data signals in various transmission systems in order that more signals could be carried by a carrier while not interfering with each other. The sidebands associated with digital signals are large, where the digital signals are of good form, and ideally the bandwidth required to handle a square wave should be infinite. This, of course, is impractical in transmission systems, and filters must be used to band-limit signals in order that they should not interfere with adjacent channels.
It has been found that there were previously no known bandwidth efficient filter designs which would meet the requirements of having both no jitter and no inter-symbol interference. The requirements of a filter have been theorized by Nyquist in well-known studies; yet despite classical design techniques using R-L-C filters, active linear filters, and transversal filters, the simultaneous achievement of no jitter and no inter-symbol interference have not been obtained.
Jitter and inter-symbol interference are among the most important parameters which contribute to the probability of error and degradation of a signal in a practical system. Even if a designer would be able to design an ideal classical Nyquist filter having no inter-symbol interference, jitter would still be present.
The filter of the present invention, on the other hand, has been found to reduce the sidebands substantially, while achieving the simultaneous benefits of no jitter and no inter-symbol interference. For example, in one successful prototype, the measured in-band to out-of-band power ratio of the signal at 1.4 times the Nyquist frequency was found to be about 20 db., and at 1.6 times the Nyquist frequency the in-band to out-of-band power was measured to be 26 db., with no jitter and no inter-symbol interference, with the input an NRZ signal. Clearly this invention is a substantial improvement over prior art filters.